Kivelson et al.: Interaction with the Jovian Magnetosphere 545 Europa’s Interaction with the Jovian Magnetosphere Margaret G. Kivelson and Krishan K. Khurana University of California, Los Angeles Martin Volwerk Österreichische Akademie der Wissenschaften Europa is embedded in Jupiter’s magnetosphere where a rapidly flowing plasma interacts electromagnetically with the moon’s surface and its atmosphere. In this chapter, the phenom- enology of the interacting system is presented and interpreted using both qualitative and quan- titative arguments. Challenges in understanding the plasma environment arise partly because of the diverse scale-lengths that must be considered as well as the nonlinear nature of the in- teractions. The discussion that follows describes selected aspects of the interacting system. On the scale of gyroradii, we describe the effects of newly ionized particles on fields and flows and their relation to wave generation. On the scale of Europa radii, we discuss the structure of the local interaction. On the scale of the tens of Jupiter radii that separate Europa from Jupiter’s ionosphere, we describe the aurora generated near the magnetic footprint of Europa in Jupiter’s upper atmosphere. We end by stressing the relevance of plasma measurements to achievement of goals of a future Europa Orbiter mission. 1. INTRODUCTION ropa, the smallest of the Galilean moons, was found also to be a plasma source (Intriligator and Miller, 1982; Eviatar The Galilean moons, although small, play a distinctive and Paranicas, 2005; Russell et al., 1999), albeit a second- role in the history of solar system science. Galileo recog- ary one. However, Europa’s plasma environment received nized that their motions in periodic orbits around Jupiter comparatively little attention until it was established by were compelling analogs of planetary bodies in a heliocen- Galileo observations that its geologically young surface lies tric system (see chapter by Alexander et al.). The complex above what is probably a global ocean (Khurana et al., orbital interactions of the inner moons were found to ac- 1998). This discovery promoted the priority of Europa and count not only for orbital stability (e.g., Goldreich, 1965), its local plasma environment as targets for further planetary but also for enhanced tidal heating (Peale et al., 1979), exploration. Although only 3 of the 12 flybys of the Galileo which powers volcanic activity on Io and melting of the ice prime mission (1995 through 1997) passed close to Europa, beneath the surface of Europa. That fluid oceans could be the next phase of the mission, designated the Galileo Europa present beneath the icy crusts of the three outer moons was mission, devoted half of its 14 flyby opportunities to Eu- discussed (Lewis, 1971) decades before spacecraft obser- ropa. Table 1 summarizes various relevant features of Gali- vations provided support for (if not full confirmation of; see leo’s flyby trajectories (or “passes”) plotted in Fig. 1. The chapter by Khurana et al.) this speculation for some of the final stage of Galileo’s odyssey included a specially designed moons, in particular, Europa. pass in which magnetometer measurements found a pre- Concurrent with studies of the interior, the particle and dicted reversal of the orientation of the internal dipole mo- fields environments of the moons began to attract attention ment, thus confirming the presence of an inductive field at following the discovery of Io’s control of jovian decametric Europa (Kivelson et al., 2000). emissions (Bigg, 1964). Goldreich and Lyndon-Bell (1969) This chapter addresses the subject of Europa’s interac- recognized that an electromagnetic link between the moon tion with the particles and fields of the jovian magneto- and Jupiter’s ionosphere could explain the observations, a sphere. The topic presents a considerable challenge because suggestion that implied the presence of plasma along the the moon and its magnetized plasma environment interact Io magnetic flux tube. Somewhat later, the existence of an nonlinearly. Relevant to the interaction are matters as di- extended nebula around Io’s orbit, the Io torus, was estab- verse as the chemical composition of the surface from lished (Kupo et al., 1976; Mekler and Eviatar, 1977). Io soon which particles are sputtered, the properties of the energetic became the focus of in situ particle and field measurements particles responsible for the sputtering, the temporal and by Voyager 1, and the ionian source of heavy ion plasma spatial characteristics of the magnetospheric plasma near that shapes the structure of much of the magnetosphere was the orbit of Europa, properties of the magnetic field that recognized (Bagenal and Sullivan, 1981; Shemansky and confines the plasma, and the electromagnetic characteris- Smith, 1981; see also review by Thomas et al., 2004). Eu- tics of the moon and its ionosphere. Europa’s response to 545 546 Europa TABLE 1. Characteristics of Galileo’s close passes by Europa (boldface emphasizes flybys with full fields and particles data with closest approach at altitudes below 2050 km). Location of Radial Distance Local Time Closest Europa Europa East c/a Relative Pass Date, Time from Jupiter (RJ) (Hours) Approach (km) Latitutde Longitude to Europa E4 12/19/96 06:52:58 9.4–9.5 16.6–17.0 688.1 –1.7 322.4 Oblique Wake E6 02/20/97 17:06:10 — — 582.3 –17.0 34.7 Recording Lost Except PWS E11 11/06/97 20:31:44 9.0–9.4 10.8–11.9 2039.3 25.7 218.7 Oblique Wake E12 12/16/97 12:03:20 9.4–9.6 14.5–14.8 196.0 –8.7 134.3 Upstream E14 03/29/98 13:21:05 9.4–9.6 14.3–14.7 1649.1 12.2 131.2 Upstream E15 05/31/98 21:12:56 9.4–9.6 9.9–10.3 2519.5 15.0 225.4 Wake E16 07/21/98 05:04:43 — — 1829.5 –25.6 133.6 Recording Lost E17 09/26/98 03:54:20 9.2–9.6 9.6–10.3 3587.4 –42.4 220.3 Wake E18 11/22/98 11:44:56 — — 2276.2 41.7 139.3 Recording Lost E19 02/01/99 02:19:50 9.2–9.4 9.7–10.0 1444.4 30.5 28.2 Upstream E26 01/03/00 17:59:43 9.2–9.7 2.8–3.1 348.4 –47.1 83.4 Upstream/Polar the currents linking it to the magnetospheric plasma in turn produces what can be thought of as a periodically varying modify the local properties of the plasma and magnetic field internal dipole moment with its axis in Europa’s equatorial (Kivelson, 2004; Kivelson et al., 2004). plane. This changing internal source contributes signifi- In the following sections we first summarize the prop- cantly to the total field near Europa (Neubauer, 1999). The erties of Jupiter’s magnetosphere in the vicinity of Europa, properties of the background plasma of the extended Io and provide a large scale (magnetohydrodynamic) perspec- torus are controlled by a combination of electromagnetic tive on the interaction between Europa, its ionosphere, and and centrifugal forces; at the orbit of Europa, the plasma its plasma environment. We then discuss the presence of density peaks between Jupiter’s magnetic equator and its pickup ions in the local plasma, the special role of ener- centrifugal equator and decreases markedly above and be- getic particles in the interaction and the modification of the low; because of the 10° tilt between Jupiter’s spin axis and interaction by the inductive magnetic field. The properties its magnetic dipole axis, plasma properties at Europa are of the inductive field itself are discussed in the chapter by strongly modulated as Jupiter rotates. The most complete Khurana et al. We consider special features of passes up- survey of plasma density near Europa (Fig. 3, taken from stream and downstream of the moon in the flowing plasma, Kurth et al., 2001) makes use of data from Galileo’s Plasma with particular emphasis on properties of the wake region. Wave System (PWS) (Gurnett et al., 1992). Power at fuh,e, Next we describe the link between Europa and the auroral the electron upper hybrid frequency, is roughly proportional footprint in Jupiter’s upper atmosphere. We close with a dis- to the square root of ne, the electron number density, with cussion of expectations for fields and particle measurements a weak dependence on |B|, the magnitude of the magnetic as a component of future exploration of the remarkable body field. The variation of density from pass to pass results that is the subject of this book. largely from the changing location of the Galileo passes relative to Jupiter’s magnetic equator, a point to which we 2. OVERVIEW OF FIELD AND PLASMA will return. CONDITIONS NEAR EUROPA’S ORBIT At Europa’s orbit, the plasma approximately corotates (meaning that it flows in the azimuthal direction at approxi- Europa’s orbit, at 9.38 RJ (RJ is the radius of Jupiter, mately Jupiter’s angular velocity) as a result of electromag- taken as 71,400 km) from the center of Jupiter and effec- netic coupling between the equatorial plasma and Jupiter’s tively in Jupiter’s equator, lies at the outer edge of the Io ionosphere. Near the equator at distances beyond ~1.4 RJ, plasma torus within the inner magnetosphere, a region where Keplerian speeds are slower than plasma rotational speeds, the magnetospheric magnetic field is quasidipolar. As Ju- so the rotating plasma overtakes bodies orbiting Jupiter. piter rotates, the 10° tilt of the dipole moment relative to Plasma flows onto Europa’s trailing hemisphere at a rela- the axis of rotation causes the magnetic equator to sweep tive speed of roughly 100 km s–1 as indicated in Table 2.